High speed machining by ultrasonic impact abrasion



Muff p 1957 c. J. THATCHER 2,804,724

HIGH SPEED MACHINING BY ULTRASONIC IMPACT ABRASION Filed Feb. 24, 1956'INVENTOR. Charles J. Thatcher.

BY%M0 ATTORNEYS.

Unite States Patent HIGH SPEED MACHINING BY ULTRASONIC manor ABRASIONCharles J. Thatcher, New York, N. Y.

Application February 24, 1956, Serial No. 567,483

7 Claims. (Cl. 5159) sued as U. S. Patents Nos. 2,736,144 and 2,736,148,

respectively, and also of my joint co-pending application with BensonCarlin, Serial No. 539,405, now U. S. Patent No. 2,774,193.

It is a particular object of the invention to make it feasible to speedup machining operations of the type hereinbefore specified, to an extentheretofore unattainable and for reasons including cooling of the toolwhich will hereinafter be explained and amplified.

The invention comprehends the rapid oscillation of comminuted hardabrasives such as aluminum oxide, silicon carbide, or boron carbide,suspended in a film or stratum of a liquid of low viscosity such aswater, and accompanied by the rapid flow of such vibrating abrasivesuspensions'and thorough scavening of the detritus formed between thesurface of the substance to be shaped and a rapidly vibrating, suitablyshaped tool; this tool may be blunt, and held with its operative surfaceconstantly adjacent to, but not usually in actual contact with, thesurface of the work piece.

The high frequency vibration of the abrasive particles is established bysonic, or preferably ultrasonic, vibration of the tool end which propelsor hammers the abrasive particles against the immediately adjacentsurface of the work piece, which is thereby subjected to myriads ofchipping actions each second; a smooth and rapid cutting of the-hardestmaterials thereby results if the impact abrasive action is not impededor rendered ineffective.

Cutting by such high-frequency vibratory impact abrasion has beenpreviously used and patented, but has been confined heretofore to thepolishing or etching of surfaces or to the drilling of holes or cavitiesor the surface, or the engraving of very hard metals, gems, glass or thelike. Thus the patent to C. H. Griss, No. 2,504,831 forAn Apparatus forEngraving Glass, patented April 18, 1950, is an example of such a priorart application of high-frequency vibratory impact abrasive action.There has been no successful attempt known to applicant, other than bythe disclosure of his before specified applications and patents, toextend the use of this novel form of abrasion to the rapid removal ofrelatively large volumes of hard materials, such as is effected inmilling machines or by the turning of such materials in lathes. Thislimitation in the application of the recently developed abrasionphenomena has, in part, been due to an insuflicient understanding of thenature of the physical-chemical phenomena involved.

I modify the prior practice of this art by making provision for a rapidand copious flow of thin, liquid abrasive slurry through the narrowlocus between the operative face 2,804,724 Patented Sept. 3, 1957 of thetool and the work confronting surface; and I further provide means forremoval of the detritus of abraded metal particles from the liquid withsufficient rapidity to maintain a return flow of abrasive slurrysubstantially free of metal detritus, so that its abrasive cuttingaction in the operative region of the device may be maintained,unimpaired, by any clogging effect of impacted abrasive particles or ofextraneous, accumulated, softer metal particles.

In the prior practice of this art it has been customary to employ verythick slurries, as much as one pound or more of comminuted boron carbidesuspended in each quart of water. Although slurries of such highconcentration have invariably caused the deposition of tenaciously cakeddeposits of abrasive even upon open, exposed plane surfaces overflowedby rapid currents of the slurry, and despite the fact that any hole orcavity exposed to even rapid flow of a normally thick slurry thereoversoon is closed and clogged by coherent plugs of impacted abrasive, itseems to have occurred to no one heretofore that the actual locus ofimpact abrasive cutting action under the tool end and-between it and inthe space over the work piece face to be abraded will likewise becomeencrusted with caked abrasive and/ or abraded particles; nor,apparently, has it heretofore been considered necessary to use means ormethods to effectually disrupt and to flush out all such impactedaccumulations of such particles or to prevent their accumulation-so asto effectually scavenge the abrasion locus. The consequences of thisoversight or neglect, and the failure to use effective correctivemeasures, has hindered and restricted the use of impact abrasive cuttingmachines and the general, widespread use of this art. It has been theaim of this applicant to locate and to apply such corrective measures,some of which have heretofore been disclosed in the co-pendingapplications and patents heretofore herein specified.

It is therefore one of the prime objects of this invention to disclose amethod specifying the optimum conditions of use of the improvedslurries-those containing additive compounds disclosed in applicantscopending application Serial No. 371,408, new issued as Patent2,736,148.

As described in said last named patent, soluble compounds,

such as sodium bicarbonate, carbon dioxide gas, or solid carbon dioxidein the form of Dry Ice, etc.,'are advantageously added to the abrasiveslurries used in impact cutting, to the end that there may be noimpoverishment of free gas bubbles to oscillate in the liquid film inthe abrasive locus at resonant frequencies, and thus forcibly to impelthe otherwise quiescent abrasive particles floating in said liquid atany moment of their transition through the film in the locus. However,suitable tests and observations of the physical and chemical phenomenaoccurring in the slurry filmv during passage of the liquid through theabrasionlocus have shown that what occurs there during irradiation, is,in fact, much more complex than it seems to have been thought to be- -asthe level of that thinking is traceable from the subject matters'ofprior art patents and literature articles-about this art. It seems thatthose who concerned themselves about the modus operandi of impactabrasion and of cutting tools involved therein, superficially consideredthat everything that occurred in the abrasion locus, could be elucidatedby the mere statement that cavitation occurred in the transient liquidfilm, and that impact abrasive cutting is controlled and its optimum usewould, and could, be attained by the mere act of ascribing it tocavitation.

Applicant has disclosed and established mathematically In his copendingapplication, Serial No. 371,408 (Patent No. 2,736,148), that resonantbubbles of gas oscillating in the liquid in the abrasion locus canincrease the kinetic energy of the liquid film as much as 14,000 timesover that attainable in gas-free irradiated aqueous slurries;

and the theoretical computations and deductions of that patent have nowbeen confirmed by suitable comparative tests. However, those tests alsomade it evident that control and establishment of factors of control,other than the concentration of free and dissolved gas in the liquid,could markedly affect the progress and etficieney of ultrasonic impactcutting. Thus, applieant has discovered, that the thermal conditions andfactors, both at the tool tip and in the liquid film beneath and incontact with that tip could and did play an important role in impactabrasive cutting.

Contrary to the generally stated assumption that this operation is onedevoid of any heat development it transpires that, in fact, there is aconsiderable generation of heat, mainly at the very tip or end of thetool, and that unless that heat is adequately dissipated as it isformed, the entire tool and the cone to which it is metallicallyattached becomes quite hot. That such heat ing should occur seems arational assumption; but the thermal conditions of the abrasive impacttool during use seems to have been assumed to be static and a matter ofno account-except that the reported apparent absence of heat in the highfrequency vibrating tool has been acclaimed as one of the virtues ofmachining methods using such tools, because the deleterious effects ofheat upon some materials have thereby been avoided.

But in various machining operations used by applicant recently, thetools became so hot duringuse that the operation was necessarilydiscontinued. This was first and most notably observed during use of thehole drilling tools disclosed in 'applicants and Benson Carlins jointapplication Serial No. 539,405, now United. States Patent No. 2,774,193.

' In the early use of that tool, the flow of slurry liquid to and fromthe locus of abrasion was very restricted,

due to what were then thought to be necessary features of toolconstruction; and applicant deduced therefrom the conclusion that acopious stream of cold liquid slurry projected upon and flowing down andover the entire exterior of the tool holder cone, should be used tosupplement the diminished cooling of the restricted flow of liquid toand from the abrasion locus.

Applicant, by this invention, accordingly uses two streams of liquidslurry in connection with each tool, instead of one single stream, asheretofore in this art. Applicants first stream is the one forcedthrough an interior channel which scavenges the detritus from theabrasion locus. Applicants second stream is a more copious stream ofcool slurry or other cooling liquid caused to flow in heat exchangingcontact with extended surfaces of the tool holder so as to cool it andthe attached tool. These two streams are illustrated in the accompanyingdrawing, which is a suitably modified copy of Fig. 12 of applicantscopending U. S. application, Serial No. 539,405, now Patent No.2,774,193.

In the drawing the figure illustrates partly in front elevation andpartly in longitudinal section, the coupled transducer 40, tapered toolholder 10, and tool 15, en gaged in drilling a hole in a work-pieceplaced in a trough 30. Extending through the tool holder and tool is achannel 24 through which slurry is forced out of the bottom of the tool15, which may be, of any suitable character depending upon the work tobe performed. Slurry is forced through the channel 24 through an inlettube 12 connected by a pipe 31 with a pump 29. A branch pipe 41 connectswith the pipe 31 and carries a nozzle 42 which discharges a stream ofcooling slurry or liquid over the outside of the tool holder. The flowof slurry discharged through the nozzle may be regulated bythe cock 43,The slurry .flows down over the large surface of the tool holder and iscollected in the trough 3t) and may berecirculated through pipe 32. "Thetool may be advanced relatively to the'work-piece in any suit- 21? IE EA EI- A hown, the Work-piece may be raised 7 4 relatively to the tool bymeans of the adjusting screw 39 under the trough 30.

But when that expedient was used, the observable exposed portion of thetool holder and toolthose portions not buried in the hole alreadyincised in the workpieceremained at or near the ambient temperature ofthe air or that of the slurry. But even then it was observed that thevery tip or end of the tool could, under some conditions of use, becomequite warm, and elevated in temperature above that of the upper portionsof the tool. This was noticed only when the tool was withdrawn quicklyfrom the embedding material of the work and before the highesttemperature of heat could be dissipated, either into the contiguous workmaterial or before that occurs, by thermal conduction, into the largemass of metal in the tool holder cone. This restricted heating of thevery tip or end of the tool is used now by applicant to good advantage,to heat the slurry film in the abrasion locus to temperatures sufiicientto evolve copious gas bubbles from the liquid slurry within any impactedlayer of abrasive or abraded particles, thus to disrupt such cakes andpermit effective scavenging of them from the abrasion locus.

. itself were constructed.

' remained comparatively and uniformly cool-if the large tool conesurface was kept constantly bathed and cooled by a copious flow ofslurry thereover, as by this invention. But tool units constructed byapplicant from stainless steel and in one piece showed a highertemperature after otherwise comparable conditions of use and heatabsorption into a full stream of slurry, which, it was deduced,'arosefrom the fact that stainless steel is a much poorer heat conductor thanMonel metal.

The considerations of the preceding several pages show that by carefuland understanding control of the physical and chemical characteristicsof the liquid film in the abrasion locus the phenomena of impactabrasive cutting may be in like manner better thereby controlled. And

7 by an application of these considerations, applicant has cake ofimpacted abrasive particles. 'such a mud-T" layer over the surface ofthe work piece devised a method to cure the before explained tendency ofthe abrasive particles of the liquid slurry to settle out to such anextent that all surfaces, even those of the abrasion locus, areencrusted with a tenaciously adhering The formation of desired to beeroded and progressively removed by im-. pact abrasion, naturally wouldhave as a concomitant effect, the cessation of effective attrition ofthe confronting work surface by impact of any of the abrasive particleswhich remained movable-from whatever cause. The impacts of minutemovable particlesthousands of them per secondwhich the ultrasonic impactabrasive technique for incisive impact cutting depends upon, will, ofcourse, become non-effective if a relatively soft layer of alreadyimpacted abrasive mud overlays and shields the brittle, hard surfaceupon which the impact bombardment is supposed to be directed.Realization of the objectives of this cutting technique consequentlypredicates the maintenance of an unprotected, bare surface of brittlematerial at all times at the lower portion of the abrasion locus; andany inception of an intervening cake of deposited and caked mud of boroncarbide or other similar, newly supplied abraded particles having cakingtendencies, will be followed by a progressive building up of furtherlayers of caked particles by the deposition and'adherence of movableabrasives which may impinge upon and enter into the initial layer; .theresult,

eventually, and in fact, almost immediately, will be building up of anunbroken encrusting mud layer, and a total cessation of the desirederosion of the confronting surface of the work-piece. V

I Consequently the scavenging of freshly abraded particles of theeroding work face from the abrasion locus must extend to include,somehow, a preventive or disrupting eifect upon the caking tendency ofthe abrasive particles which are relied upon to. effect the erosion. Ifremoval of the accumulated abrasive and abraded particles does not keepstep with the increase of such particles in the locus, then the cuttingprocess becomes throttled, impeded and self-destructive.

This explains why the films, as the slurry passes through the locus ofabrasion, should become warm enough to evolve carbon dioxide bydissociation of sodium bicarbonate or of HCO's ion into carbon dioxidebubbles; for liberation thereof within any layer of impacted abrasion orabraded particles, will disrupt such a cake and permit effectivescavenging of detritus from the locus of abrasion.

Applicant, in this invention, also uses a method of amplification of thepower impulses, sonically generated by the transducer, which is anequivalent means of accomplishing the hydraulic amplification disclosedin Serial No. 321,579 (now Patent No. 2,736,144). This amplifying meanscan obviously be a tapered tool holding cone such as that described inthe U. S. patent to Noyes No. 2,044,807 issued June 23, 1936, or in theU. S. patent to Smith No. 2,407,294 issued September 10, 1946. This kindof amplification is also described in the U. S. patent to Carwile No.2,651,148 and to Calosi, No. 2,632,858 assigned to Raytheon Mfg. Co.This means of amplification is accordingly well known in the art andrequires no further consideration except to state that when used, it iscomprehended by the claims of applicants issued patents and those of hisinstant application, in combination with other elements thereinspecified.

Using a transducer and such tapered power amplifying tool holders,vibrations of the tool of amplified amplitude and the high frequency ofthe transducer, accompanied by movement of the work in a plane divergentto that of the axis of movement of the vibrating tool, produces a rapidcutting of the surface of the work along the line of contact of the toolface and the work; the vibration need only be of the order of a fewthousandths of an inch in amplitude, and to have a frequency from fourhundred to thirty thousand cycles per second, or even more, or fromeight hundred to sixty thousand motions per second, or more, to producethe desired result. It will be understood by those skilled in the artfrom the foregoing description, that these vibrations may be produced bya number of different means and that they may be imparted to either thetool or to the work, or to both.

It will also be understood that the slurry to be circulated thru theinterior of the tube, then underneath the tip of the tool and thenupwardly to the surface of the incision, should have a suitableconsistency for the nature of the machining to be done; this may be, forexample, a suitable thin liquid slurry, such as water containing finelycomminuted abrasive, such as one pound of boron carbide in suspensionper gallon of water. It has heretofore been the usual practice to useone' pound of abrasive in each quart of water, which practice, I find,tends to increase the caking tendency and deposition of mud and toimpede the circulation of the slurry.

By use of the method and means herein disclosed, employing the describeddepartures from prior art practices, applicant has incised and shapedhard brittle work materials at speeds many times higher than thoseheretofore attainable.

I claim:

1. The method of removing material from a hard substantiallynon-yielding solid by vibratory impact abrasion, which comprisesimparting high frequency oscillations to the end of a blunt faced toolheld against rotation by a tapered, elongated tool holding memberadapted to amplify the acceleration of said vibratory impact, applyingsaid tool end to the work surface of said solid with the cutting fact ofthe tool forming an angle of not less than 45 to the plane of saidsurface, flowing over the surfaces of the holder and tool andinterposing in the 10- cus of abrasion between the. tool end and worksurface, a film of a thin liquid suspension of a mobile, finelyconiminuted abrasive containing in solution a substance whichdissociates, when said film is irradiated and heated, into pulsating gasbubbles, scavenging mobile and impacted abrasion and abraded particlesfrom said locus while heating said film in said locus by absorption ofheat generated in the end surface of said tool during the machineoperation, and controlling the temperature of said tool end surface bycontrolling the rate of flow of said liquid over said tool holder andtool, and advancing the tool into the work as the abrasive progressivelyremoves material from the contact area.,

2. The method of removing material from a hard substantiallynon-yielding solid by vibratory impact abrasion, which comprisesimparting high frequency oscillations to the end of a blunt faced toolheld against rotation by a tapered, elongated tool holding memberadapted to amplify the acceleration of said vibratory impact, applyingsaid tool end to the work surface of said solid with the cutting face ofthe tool forming an acute angle to the plane of said surface, flowingover the surfaces of the holder and tool and interposing in the locus ofabrasion between the tool end and work surface, a film of a thin liquidsuspension of a mobile, finely comminuted abrasive containing insolution a substance which dissociates, when said film is irradiated andheated, into pulsating gas bubbles, scavenging mobile and impactedabrasion and abraded particles from said locus while heating said filmin said locus by absorption of heat generated in the end surface of saidtool during the machine operation, and controlling the temperature ofsaid tool end surface by controlling the rate of flow of said liquidover said tool holder and tool, and {advancing the tool into the Work asthe abrasive progressively removes material from the contact area.

3. The method of removing material from a hard substantiallynon-yielding solid by vibratory impact abrasion, which comprisesimparting high frequency oscillations to the end of a blunt faced toolheld against rotation by a tapered, elongated tool holding memberadapted to :amplify the acceleration of said vibratory impact, applyingsaid tool end to the work surface of said solid with the cutting face ofthe tool forming an obtuse angle to the plane of said surface, flowingover the surfaces of the holder and tool and interposing in the locus ofabrasion between the tool end and work surface, a film of a thin liquidsuspension of a mobile, finely comminuted abrasive containing in asolution a substance which dis sociates, when said film is irradiatedand heated, into pulsating gas bubbles, scavenging mobile and impactedabrasion and abraded particles from said locus while heating said filmin said locus by absorption of heat generated in the end surface of saidtool during the machine operation, and controlling the temperature ofsaid tool end surface by controlling the rate of flow of said liquidover said tool holder and tool, and advancing the tool into the work asthe abrasive progressively removes material from the contact area.

4. The method of removing material from a hard substantiallynon-yielding solid by vibratory impact abrasion, which comprisesimparting high frequency oscillations to the end of a blunt faced toolheld against rotation by a tapered, elongated tool holding memberadapted to amplify the acceleration of said vibratory impact, applyingsaid tool end to the work surface of said solid with the cutting face ofthe tool being parallel to said surface, flowing over the surfaces ofthe holder and tool and interposing in the locus of abrasion between thetool end and work surface, a film of a thin liquid suspension of amobile, finely comminuted abrasive containing in solution a substancewhich dissociates, when said film is irradiated and heated, intopulsating gas bubbles, scavenging mobile and impacted abrasion andabraded particles from said locus while heating said filmin said locusby absorption of heat generated in the end surface of said tool duringthe machine operation, and controlling the temperature of said tool endsurface by controlling the rate of flow of said liquid over said toolholder and tool, and advancing the tool into the work as the abrasiveprogressively removes material from the contact area.

5. The method of removing material from a hard, substantiallynon-yielding, solid material, by vibratory impact abrasion, whichcomprises vibrating at high frequency, positioning and maintaining theend of a hollow, blunt-ended tool in proximity to the confrontingsurface of the work material which is to be removed, holding said toolagainst rotation by a tapered, channeled and elongated tool-holdingmember connected at its larger end to a source of high frequency, lowamplitude axial vibrations and, at its smaller end to the non-operativeend of said tool, thus to amplify the acceleration of impacts of saidtool end upon the locus of abrasion, impelling into and forcing, throughthe interior of said hollow tool and into and away from said locus andtool end, a stream of a liquid suspension of a comminuted abrasive thusto scavenge abraded detritus from said locus and tool end, and, also,flowing another stream of said liquid suspension over the outersurfaces, of said tool-holding member and its attached tool, thus tocontrol and remove heat of abrasion developed in said tool end andlocus.

6. A vibrating device for high speed machining by high frequency impactabrasion, which comprises a source of high frequency mechanicalvibrations, means for trans.-

mitting and for increasing the amplitude of said vibrations, comprising,an elongated medium attached to said source at oneend of said medium, amachining tool attached thereto, at its other end, and means forceablyto how a stream'of alliquid suspension of a comminuted abrasive to,through andtaway from the-locus of abrasion between said tool end and anadjacent work-piece surface, thus to scavenge the abrasion detriutusfrom said locus andtool end, in combination with means to flow anotherstream of-cooling liquid in contact with said elongated medium, thus toreduce the heat of abrasion developed'by said high-speed machiningoperation.

7. A high-speedultrasonic machining method for removal of material byimpact abrasion, from a hard substantially non-yielding solid, whichcomprises: vibrating an elongated tool holder connected at one end to asource of high frequency oscillations, connecting a tool to the otherend of said tool holder, holding the tool end adjacent to the surface ofthe solid as it is machined, forceably flowing a stream of a liquidsuspension of a comminuted abrasive to, through and away from the locusof abrasion between said tool end and said surface, thus to scavengedetritus of abrasion from said locus, and, also, flowing another streamof cooling liquid in heat exchanging contact with said tool holder, thusto reduce tool heat generated by said high-speed machining.

. References Cited in the file of this patent UN IIED STATES PATENTS2,044,807 Noyes June 23, 1936 2,407,294 Smith Sept. 10, 1946 2,632,858Calosi, Mar. 24, 1953 2,651,148 Carwile Sept. 8, 1953 2,736,144 ThatcherFeb. 28, 1956 2,736,148 Thatcher Feb. 28, 1956

